A heliostat is a mirror-like reflective surface, intended to reflect solar radiation and direct it towards a specific target.
These heliostats are adjustable and are normally provided with a two-axis tracking system to follow the sun individually on two axes. These drives enable the heliostat to achieve azimuth and elevation movement.
In thermoelectric central receiver power plants, the heliostat forms part of the solar field. The solar field consists of a series of heliostats, the main aim of which is to reflect solar radiation and direct it toward a receiver situated at the top of a tower. Therefore, the heliostat normal is always halfway between the sun and the tower receiver.
Inside the receiver is a fluid which is heated by the solar radiation and is either passed through a turbine directly to generate electricity or it is used afterwards in a heat exchanger to produce vapour which will finally be passed through a turbine.
The heliostat therefore, is an element that has a reflective surface, a support structure, an azimuth and elevation drive mechanism, a pedestal, with its corresponding foundations and a local control system.
Various types and configurations of heliostats have been developed over time.
Within the state of the art we can find heliostats with a continuous reflective surface, heliostats using Fresnel reflecting system, a stretched membrane heliostat or a faceted heliostat.
Heliostats with a continuous reflective surface can have completely flat mirrors, both in terms of structure and reflective surface, such as the case of the heliostats of patent US2009/0007901, which we shall discuss later on.
Stretched membrane heliostats are mainly made up of a circular structure and a slightly curved stretched membrane, with a reflective surface where the solar energy is concentrated.
This curvature is what enables the heliostat to concentrate the sun's radiation.
With regard to the faceted heliostat, in this case the heliostat consists of a number of smaller reflective elements, together with a structure, which we shall call facets and which all together form the final heliostat when the facet is arranged in a specific orientation (known as “canting”).
With regard to faceted heliostats, there are a number of configurations for the arrangement of these facets in the state of the art. U.S. Pat. No. 4,276,872, through its drawings, describes a faceted heliostat, where the facets that make up the heliostat, have a structure or support as well as a flat reflective surface.
In order to provide faceted heliostats with the ability to concentrate the sun's radiation and thus get the maximum thermal solar power radiation into the receiver, the reflective surface of the facets can be curved.
Patent ES2351755 describes a system used to manufacture heliostat facets, made up of a reflective surface and a support where the reflective surface is curved beforehand.
Patent ES2326586 describes a facet for a heliostat, configured from a flat structure, in which the reflective body is also curved.
Depending on the shape of the reflective surface, there are two types of heliostats: Non-imaging focusing heliostats and imaging focusing heliostats.
) Non-imaging focusing heliostats such as the one proposed in “Non-imaging focusing heliostat” —Y. T. Chen , K. K. Chong, T. P. Bligh , L. C. Chen, Jasmy Yunus, K. S. Kannan, B. H. Lim, C. S. Lim, M. A. Alias, Noriah Bidin, Omar Aliman, Sahar Salehan, SHK. ABD. Rezan S.A.H., C. M. Tam and K. K. Tan., enable the astigmatism effect to be reduced, the effect of optical aberration, which can be translated into the lack of power in the receiver for a cavity aperture attached beforehand and which occurs when the incidence angles (angle of the heliostat's normal to the incident beam which ranges from 0 to 90°) are different from zero. Unfortunately there is a cost issue with this type of heliostat because the tracking is rotation-elevation: with one of the heliostat's main axes always remaining perpendicular and the other always parallel to the flat plane formed by the incident beam, the heliostat's normal and the reflected beam. These two rotation-elevation movements mean the heliostat's centre of gravity does not remain in line with the pedestal, which involves more complex structures than normal. Also, additional mechanisms are required for the different facets, given that each of them rotate independently from one another, so an axis orientation can be achieved that directs the beams to the centre of the receiver.
In other execution methods, the heliostats are Fresnel-type reflectors with an azimuth and altitude-type tracking system. This geometry has the disadvantage of limited movement ranges (patent ES1074545U) and second order cosine effects that counteract the savings related to eliminating the curvature of the structure.
2) Furthermore, imaging focusing heliostats include flat heliostats and spherical or revolving parabolic heliostats.
Flat heliostats, for a defined design acceptance β(angle that takes into account the different errors related to the manufacturing and assembly of the heliostat as well as the angle subtended by the sun) do not have a concentration capacity and they project their own aperture (mirror surface) on the receiver, amplified linearly in distance by the acceptance angle. If large sizes were used, the losses as a result of an overflow on to the receiver aperture (losses due to the amount of radiation reflected by the concentrator and not reached by the receiver) for isolated heliostats would make the investment made for the heliostat itself unfeasible.
These flat and small heliostats do not concentrate but they enable modular plants to be constructed with a significant reduction in structural and foundation costs and savings on the curvature of the mirrors, however, it does mean that operating and maintenance costs increase, given that more mechanisms, control and related procedures are required in order to provide the same thermal power as we could generate with fields with larger heliostats.
These fields with flat and small heliostats are described in the aforementioned patent US2009/007901.
For heliostats with a reflective rotating parabolic surface, the optical concentration is different: defined design acceptance and for practically zero solar radiation incidence angles, this type of collector maximises the concentration when the focal distance is around 0.6 times the aperture with the concentration reached being C=0.25 (1/sen2δ)=0.25 Cmax; i.e., 0.225 times the maximum concentration possible for the optical in question, where δ is the semi-angle of acceptance β=2 δ. In practice the heliostats of spherically formed tower plants have a much higher focal distance than the optimal one under the criteria of maximum capture Cmax=1/sen2δ; which means that, with small or almost zero incidence angles, the heliostat concentrates less the further away it is from the receiver, with the heliostat failing to concentrate at a specific distance, i.e., the generated spot reflected on the receiver is larger than the heliostat's actual aperture and only considering the rays that are inside the collector's design acceptance. It is important to point out that, for small incidence angles and normal focal distances in the tower plant heliostat fields, the parabolic geometry is similar to the spherical geometry. Therefore taking the geometry of spherical heliostats, for these or greater focal distances, the size of the spot on the receiver is the same aiming with a large heliostat or with a small one with the same curvature always for almost zero incidence angles. The difference between the two appears for incidence angles greater than zero (incidence angle is the angled formed by the vector that directs the incident beam on a point of the heliostat, with the normal to the reflective surface on said point), so for large surface spherical concentrators, a more pronounced astigmatism effect occurs, which leads to a large and more wide-spread image.
Based on the focal distance of the heliostat (f) defined as the distance from the heliostat pivoting point (centre of the reflective surface) to the focal point situated on top of the tower f=√{square root over ((X−X0)2+(Y−Y0)2(Z−Z0)2)}; the radius of the curvature of the heliostat in question is defined as R theoretical=5f, with R=2√{square root over ((X−X0)2+(Y−Y0)2(Z−Z0)2)} where X, Y, Z are the heliostat positioning coordinates and (X0, Y0, Z0) are the focal point coordinates. It is important to point out that tower plants with this type of large surface parabolic or spherical heliostats have a number of different curvatures in the field: i.e., each heliostat has a different curvature depending on its positon in the solar field and its distance from the tower.
These types of spherical heliostats are also more difficult to assemble and manufacture as a result of having to curve the facet mirrors, then having to cant them on the heliostat (orientate the facets on the heliostat structure so the final shape is parabolic or a perfect sphere) and cant the structure, i.e., configure it with the curvature and desired shape depending on its position with regard to the tower. However, it has the advantage of lower operating and maintenance costs, maintaining low structural, control and instrumentation and mechanism costs.
3) In patent ES8306688A1 it already mentions the possibility of the reflective surface of the heliostat facets being flat, incorporating the mirror supports, to regulate the orientation of these around the vertical and horizontal axes.
The invention at hand, intends to bring together the advantages in one solar field with both flat facet heliostats and heliostats with parabolic or spherical surfaces and a mixture of both optimising the size, the total cost and the distribution on the plant if applicable.